1. Field of the Invention
This invention relates to a circuit and a method for processing received signals. One implementation of the invention is in a receiver system that combines a number of multi-path signal components delayed by differing amounts of time before reaching the receiver. Such a system is typically found in a code-division multiple-access (CDMA) demodulation system, or “rake receiver”.
2. Description of the Related Art
Code-division multiple-access (CDMA) systems have been in existence for some time in military communications networks and the like, but have more recently gained popularity in more general telecommunications networks. For example, international standards specification IS-95 (Mobile Station—Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System. TIA/EIA-95) is based on a CDMA system, and public mobile telecommunications networks operating in accordance with IS-95 are now in operation in the USA, South Korea, Hong Kong, and a number of other countries.
The first-generation analogue based mobile telecommunication systems originally used a pair of dedicated radio channels with typically 30 kHz bandwidth to establish a connection between the base station (BS) and mobile station (MS). Second generation digital systems typically operate according to a time division multiple access (TDMA) regime, such as in the case of the Global System for Mobile Communication (GSM) and IS-136 systems. In a TDMA approach, each pair of dedicated radio channels is divided into a number of timeslots to enable communications between the base station and a plurality of mobile stations, with each mobile station being allocated a separate timeslot on the channel. With a CDMA system the radio channel bandwidth is typically larger than for an analogue system, for example 200 kHz channel bandwidth for GSM.
CDMA systems implement a form of spread spectrum communications, and employ a pair of radio channels with a much larger bandwidth, such as 1.5 MHz for IS-95 or 5.0 MHz for UTRA FDD (Universal mobile telecommunications system Terrestrial Radio Access—Frequency Division Duplex). The bandwidth of each CDMA channel can simultaneously convey multiple separate communications in a manner which is neither frequency divided nor time divided as in the case of the analogue systems and TDMA systems, respectively. Basically, each user is assigned a pseudo-noise (PN) random sequence (constructed from channelization codes and scrambling codes) which is used to modulate the information (e.g. symbols) to be conveyed onto the channel. If the receiver has knowledge of the PN code, it is able to distinguish one user from the others and recover the relevant information.
CDMA systems make use of a PN code that has a much higher bit rate than the symbol-rate of the information being transmuted. The bit-rate of the PN code is referred to as the chip rate. The chip rate is typically 2n (3, 4, 8, 16, 32, . . . ) higher than the symbol rate. After modulation with the PN code, the resulting chip sequence is passed through a filter with a root-raised cosine frequency characteristic. The signals output from the filter are then presented to the radio frequency (RF) system for modulation onto a RF carrier.
In a wireless communication system, the signals transmitted from the base station (BS) usually travel across a certain distance before reaching the mobile station (MS). When passing through the air interface, apart from the direct path between the BS and MS, there may additionally be longer paths as a result of the signal being reflected by buildings, the ground and other large objects. Hence, at the MS it is not unusual to receive a number of signals, possibly having different received power levels and time delays. These replicas of signal are referred to as multi-path signals. The time difference between the first and last reaching the MS is generally referred to as the delay spread.
In a narrow-band wireless communication system, multi-path signals generally create extra interference. However, for wideband systems such as W-CDMA (Wideband Code Division Multiple Access), the use of advanced receiver designs can be used to resolve the multi-path signals and eliminate the negative effects of the interference. When the received signal is demodulated from the RF carrier back to a baseband signal, a further demodulation is necessary to remove the PN sequence before the original symbol information can be retrieved. A plurality of baseband demodulators (e.g. 4-6) can be employed with each one assigned to demodulating the PN sequence from the received baseband signal based on different transmission delays. These baseband demodulators are referred to as “rake fingers”.
The final stage in the demodulation process (performed by a combiner) involves combining the outputs from each of the rake fingers constructively to arrive at the received symbol sequence. The use of a number of rake fingers to demodulate a number of signals received from the air-interface improves the decoding probability of the received sequence. Thus, the utilization of rake fingers and combiner not only overcomes the multi-path interference, it also serves to improve the quality of the de-spread signals.
As long as the delay spread is significantly smaller than the symbol period, the outputs from the rake fingers can be easily combined using a simple addition. However in the UTRA 3G FDD CDMA system, this is no longer the case. In that case, the chip rate employed is 3.84 Mbits/sec, and the symbol rate can range from 960 kbits/sec to 15 kbits/sec. Considering the higher symbol rate of 960 kbits/sec yields a duration of 1.04 μs. It is not unusual for a delay spread to be of the order of 10 μs.